Investigation of the structural conditionality for changes in physical properties of K3H(SO4)2 crystals

Solid acid proton conductors: the influence of hydrogen subsystems on physical properties

“…The occurrence of high superprotonic conductivity in the M 3 H(XO 4 ) 2 crystals is associated with the formation of a qualitatively new and dynamically disordered hydrogen-bond system. This fact was found for the first time in the studies of structural phase transitions in the Rb 3 H(SeO 4 ) 2 crystals [1] and was then confirmed for other M 3 H(XO 4 ) 2 crystals, including K 3 H(SO 4 ) 2 [2]. In K 9 H 7 (SO 4 ) 8 •Н 2 О crystals, the only known representative of the M 9 H 7 (XO 4 ) 8 •xН 2 О family, the superprotonic phase transition at 405 K is associated with the diffusion of crystallization water, the hydrogenbond network rearrangement, and the formation of channels for the possible motion of K atoms [3,4].…”

A comparison of the Ca3(PO4)2 and CaSiO3 systems, with a new structure refinement of tuite synthesized at 15 GPa and 1300 C

“…Several palmierite-type compounds are capable of incorporating H atoms into their structures as an essential component, such as previously observed for Cs 3 H(SeO 4 ) 2 (Merinov et al 1990;Sonntag et al 1998), Rb 3 H(SeO 4 ) 2 (Baranov et al 1987;Magome et al 2009), Tl 3 H(SO 4 ) 2 (Matsuo et al 2002), and K 3 H(SO 4 ) 2 (Makarova et al 2010). This, thus, calls it into question whether tuite can accommodate any H atoms during its formation from hydroxylapatite and whitlockite, ideally Ca 9 Mg(PO 4 ) 6 (PO 3 OH), at high temperatures and pressures.…”

Superprotonics—crystals with rearranging hydrogen bonds

“…In comparison with other isostructural crystals, K 3 H(SO 4 ) 2 samples exhibit anomalously slow kinetics; therefore, it was impossible to detect the phase transition using dynamic methods such as dif ferential scanning calorimetry, impedance spectros copy, and polarized light observations upon continu ous heating. To detect a transition and to correctly determine its temperature, measurements should be performed in step heating mode with prolonged hold ings at a constant temperature [29,30]. As deter mined, the superprotonic phase in these crystals exists in a small range; as the temperature increases, solid phase reactions begin in samples (while retaining the crystalline state).…”

“…All these factors complicated the detection of the high temperature superprotonic phase and the determination of its structure. X ray diffraction, scanning electron microscopy, and X ray energy dispersive microanalysis studies [30] of sample subjected to heating showed the existence of K 2 SO 4 and K 2 S 2 O 7 phases which can be formed as the temperature increases due to the reaction Initially, heating of K 3 H(SO 4 ) 2 crystals leads to the hydrogen bond system transformation: a dynamically disordered system of bonds between oxygen and hydrogen atoms is formed; upon further heating, oxy gen atoms with two nearest hydrogen atoms are broken from SO 4 tetrahedra, forming a water molecule which diffuses from the crystal. The experimental data obtained suggest that a multiphase state, i.e., a com posite, is formed in crystalline samples in a certain temperature range, in which conductive and noncon ductive crystalline phases coexist [30].…”